The proposed project, submitted in response to PA-04-085 will study cognitive and behavioral functioning in children with DSM-IV Autistic Disorder who also manifest symptoms of Attention Deficit Hyperactivitv Disorder (ADHD). Although the DSM-IV does not specifically allow for a comorbid diagnosis of ADHD with Autism, many children with Autism manifest symptoms of ADHD, and many are treated for it. There have been few studies on ADHD in Autism, so little is known about its cognitive and behavioral manifestations, or the optimal parameters for treatment. The proposed project will address these issues by examining cognitive and behavioral functioning in children with Autism who manifest symptoms of ADHD, as well as their response to stimulant medication treatment. Building upon techniques developed in the PI's previous NIMH-funded studies (MH044118 and MH48212, DA Pearson, PI) as well as her collaborative work in a current NICHD autism program project (HD035471; KA Loveland, PI; DA Pearson, Co-lnv.), this proposed study will address two Specific Aims. In Specific Aim 1, cognitive and behavioral functioning as well as comorbid psychiatric symptomatology will be compared in: 1) children with Autism who manifest symptoms of ADHD ("Autism/ADHD"), and 2) children with Autism who do not manifest symptoms of ADHD ("Autism/non-ADHD"). In both groups, children will be 7-12 years old, with an IQ of 50-130. It is hypothesized that relative to children, children in the Autism/non-ADHD group, with Autism/ADHD will have greater deficits in attention and inhibition, as well as more internalizing and externalizing psychiatric symptomatology. In Specific Aim 2, the cognitive and behavioral response to placebo, low, medium, and high doses of methylphenidate (MPH) in children with Autism/ADHD will be studied in a medication treatment trial, using a double-blind, placebo-controlled, crossover design to determine optimal MPH dosages. It is hypothesized that higher MPH doses will produce successive improvements in cognitive and behavioral functioning in children with Autism/ADHD. Thus, the primary goals of the proposed research are to characterize ADHD in school-aged children with Autistic Disorder (DSM-IV), and to assess their cognitive and behavioral response to MPH.